Thompson Faces Hurdles in Desire for Runoff

Unions Unite Behind de Blasio as Democratic Candidate for Mayor

Bill Thompson faced a new series of obstacles Wednesday in his New York City mayoral bid, as key unions threw their support to Public Advocate Bill de Blasio, whose margin in the Democratic primary grew slightly.

By Wednesday afternoon, Mr. de Blasio sat on a cushion of 2,000 votes above the 40% threshold he needed to avoid a runoff with Mr. Thompson, according to unofficial tallies by the city Board of Elections. Mr. Thompson had...